Factors controlling free thermal convection in faults in sedimentary basins: implications for the formation of zinc–lead mineral deposits

Stratiform sediment‐hosted Zn–Pb–Ag mineral deposits constitute about 40% of the Earth's zinc resources ( Allen 2001 ), and in most cases their genesis involves the discharge of basinal brines near or on the seafloor through syndepositional faults ( Sangster 2002 ). From the point of view of base metal exploration, it is therefore essential to identify all possible faults that formerly carried the upwelling ore‐forming solutions during mineralising events. This paper presents a numerical investigation of the relative importance of various physical parameters in controlling fluid discharge, recharge and heat transport in faults. A two‐dimensional, free convection of pure water, hydrogeological model is developed for the McArthur basin in northern Australia based on the surface geology, known stratigraphic and structural relationships and regional geophysical interpretations. Numerical experiments and sensitivity analyses reveal that faults with strong initial heat input, due to depth of penetration or magmatic activity, are the most likely candidates to carry discharge fluids to the sites of metal precipitation. Deeper, wider and more permeable faults are more likely to behave as the fluid discharge pathways, whereas shallow, narrow or less permeable faults act as marine water recharge pathways. Compared with these fault‐related factors, aquifer physical properties are less important in determining fluid flow patterns and the geothermal regime. These results are an important step in understanding hydrothermal fluid flow in sedimentary basins in order to develop effective exploration criteria for the location of stratiform Zn–Pb–Ag deposits.     

From Boatyard to Museum: 3D laser scanning and digital modelling of the Qatar Museums watercraft collection,Doha, Qatar

This article presents the results of a project to 3D laser scan and digitally model 14 watercraft from the Qatar Museums collection, comprising a range of regional vessels: most had not been surveyed previously. The project used the resulting point clouds generated 2D naval lines and orthographic records of the vessels in their current condition, and photorealistic 3D digital models for gallery display. This case study provides illustrative examples of the intermediate stages and final outputs. It assesses the pros and cons of 3D laser scanning as a survey technology for nautical scholars in terms of the time, cost, and skillset, as well as logistical considerations. It also compares the accuracy of traditional hand survey methods.     

3D heritage visualisation and the negotiation of authenticity: the ACCORD project*

This article examines the question of authenticity in relation to 3D visualisation of historic objects and monuments. Much of the literature locates their authenticity in the accuracy of the data and/or the realism of the resulting models. Yet critics argue that 3D visualisations undermine the experience of authenticity, disrupting people’s access to the materiality, biography and aura of their historic counterparts. The ACCORD project takes questions of authenticity and 3D visualisation into a new arena – that of community heritage practice – and uses rapid ethnographic methods to examine whether and how such visualisations acquire authenticity. The results demonstrate that subtle forms of migration and borrowing occur between the original and the digital, creating new forms of authenticity associated with the digital object. Likewise, the creation of digital models mediates the authenticity and status of their original counterparts through the networks of relations in which they are embedded. The current pre-occupation with the binary question of whether 3D digital models are authentic or not obscures the wider work that such objects do in respect to the cultural politics of ownership, attachment, place-making and regeneration. The article both advances theoretical debates and has important implications for heritage visualisation practice.     

Numerical modelling of 3D fluid flow and oxygen isotope exchange in fractured media: spatial distribution of isotope patterns

An understanding of fluid flow, mass transport and isotopic exchange in fractured rock is required to understand the origin of several geological processes including hydrothermal mineral deposits. The numerical model HydroGeoSphere simulates 3D advection, molecular diffusion, mechanical dispersion and isotopic exchange in a discretely fractured porous media, and can be used to better understand the processes of mass transport and isotopic exchange in fractured rocks. Study of ¹⁸O isopleth patterns for different types of fractures and fracture networks with a range of structural complexity and hydraulic properties shows that fracture properties and geometry control mass transport and isotopic exchange. The hydraulic properties, as well as the density, spacing, and connectivity of fractures determine the isotopic patterns. Asymmetries in the geometry of oxygen isotope patterns could be used to determine the direction of hydrothermal fluid flow.     

Geothermometry and geobarometry of overpressured environments in Qiongdongnan Basin,South China Sea*

We demonstrate the use of PVT fluid inclusion modelling in the calculation of palaeofluid formation pressures, using samples from the YC21‐1‐1 and YC21‐1‐4 wells in the YC21‐1 structural closure, Qiongdongnan Basin, South China Sea. Homogenisation temperatures and gas/liquid ratios were measured in aqueous fluid inclusions, and associated light hydrocarbon/CO₂‐bearing inclusions, and their compositions were determined using a crushing technique. The vtflinc software was used to construct P–T phase diagrams that enabled derivation of the minimum trapping pressure for each order of fluid inclusion. Through the projection of average homogenisation temperatures (155, 185.5 and 204.5°C) for three orders of fluid inclusion on the thermal‐burial history diagram of the Oligocene Yacheng and Lingshui formations, their trapping times were constrained at 4.3, 2.1 and 1.8 Ma, respectively. The formation pressure coefficient, the ratio of fluid pressure/hydrostatic pressure established by PVT modelling coupled with DST data, demonstrates that one and a half cycles of pressure increase–discharge developed in the Yacheng and Lingshui formations for about 4.3 Ma. In comparison, the residual formation pressure determined by 2D numerical modelling in the centre of LeDong depression shows two and a half pressure increase–discharge cycles for about 28 Ma. The two different methods suggest that a high fluid potential in the Oligocene reservoir of the YC21‐1 structure developed at two critical stages for regional oil and natural gas migration and accumulation (5.8 and 2.0 Ma, respectively). Natural gas exploration in this area is therefore not advisable.     

New insights from reactive transport modelling: the formation of the sericitic vein envelopes during early hydrothermal alteration at Butte, Montana

A reactive transport computer code has been employed to model hydrothermal alteration of a granitoid rock bordering a discrete vein channel. The model suggests that the grey sericitic and sericitic with remnant biotite alteration envelopes at the porphyry copper deposit at Butte, Montana, can be formed by a reducing, low pH, and low salinity fluid under constant temperature and pressure conditions of approximately 400 °C and less than 100 MPa during a time span of approximately 100 years or less. Hydrothermal alteration has little effect on the porosity of the host rock (Butte Quartz Monzonite), and the diffusivity of the aqueous species also changes little. A sequence of mineral reaction fronts characterizes the alteration envelopes. The biotite dissolution front occurs closest to the vein channel and marks the transition from the grey sericitic to sericitic with remnant biotite envelope. The plagioclase dissolution front occurs farthest into the matrix and marks the edge of relatively fresh Butte Quartz Monzonite. From the properties of the quasi‐stationary state approximation ( Lichtner 1988 ; Lichtner 1991 ), it follows that once the sequence of reaction fronts is fully established, their relative locations remain constant and the widths of the reaction zones increase with the square root of time.     

Studying Human Responses to Environmental Change: Trends and Trajectories of Archaeological Research

ABSTRACT

Environmental archaeological enquiry has a long and vibrant history. Many of the same questions have persisted in archaeological dialogues over the past century. In particular, the effects of environmental change on demographic patterns, health, and societal stability are among the most pervasive questions being addressed by anthropological research. These studies have limitations, however. For example, evaluations of the complex relationships between environmental variables and human responses are only just beginning to emerge in anthropological literature. This goal requires high-resolution paleoclimate datasets and the use of quantitative modelling rooted in evolutionary and complex systems theory. This paper serves as a broad review of advances in environmental archaeological enquiry associated with environmental change and human response. I argue that the future of archaeological questions concerning human-environmental connection requires a re-evaluation of causality and the incorporation of complex systems approaches to address human responses to external pressures.     

Hydrology at Aberdeen – thinking about water locally and globally

ABSTRACT

Although geographical hydrology research at Aberdeen contributed significant insights into the connections between river channel processes and landscape evolution in the early twentieth century it was not until the 1980s that hydrology became a discernible research theme in the Department of Geography. Whilst the research initially focused on understanding the linkages between fluvial geomorphology, hydrology and in-stream hydraulics, since the mid-1990s a burgeoning research community has made significant contributions to advancing physical hydrology, understanding water quality issues and elucidating inter-connections between hydrology and both terrestrial and aquatic ecosystems. A particular research strength has been the use of isotopic tracers (especially high-resolution, field-based data) to advance hydrological models. Work at Aberdeen has also been internationally leading in terms of understanding the role of hydrology in shaping the life cycle of Scottish Atlantic salmon and other aquatic organisms. Ideas and research approaches developed at local Scottish field sites are being increasingly applied and adapted to advance hydrological understanding in geographical regions as diverse as the rain forests of Costa Rica, the karst of south west China, aquifers in east Africa, the Chilean high Andes, Arctic Canada, peatlands in western Siberia and forests in northern Germany.     

The Power of Models in Planning: The Case of DaisyGIS and Nitrate Leaching

Spatial deterministic simulation models such as DaisyGIS are increasingly being used for decision‐making in European planning. In Denmark specifically, there is an increasing interest at the regional level to use DaisyGIS as an administrative tool. There are, however, uncertainties associated with the use of these deterministic models, partly due to the spatial variation in input parametres. This study evaluates the use of DaisyGIS for nitrate leaching assessments based on nitrate leaching experiments carried out at the Marbjerg experimental site at Roskilde University. The study includes geostatistical analysis of soil and hydraulic variables and the use of remote sensing for deriving spatial information about the Leaf Area Index. It is found that the model predicts nitrate leaching well when using site‐specific input data but it is quite sensitive to the parameterization of carbon, the C/N ratio and variations in the Leaf Area Index (LAI). With the increasing use of Daisy in the counties, there is a need to find better ways of handling uncertainties in model output which can be used by decision‐makers and to study further the impact of spatial LAI assimilation on catchment scale nitrate leaching processes.